The proposed research in the treatment of children with acute lymphocytic leukemia has the following objectives: 1) To determine in a controlled manner the relative efficacy and toxicity of two therapeutic regimens given early in remission for prophylaxis of central nervous system leukemia; 2) To determine the efficacy and toxicity of periodic "reinforcement" or "reinduction" chemotherapy in prolonging continuous complete remission; 3) To prolong the duration of leukemia-free survival with combination chemotherapy and preventive central nervous system therapy; 4) To analyze results in this to earlier studies to determine the reasons for differences, despite no apparent differences in therapy; 5) To continue studies of surviving patients for an indefinite period of time to detect late toxic effects of therapy such as growth impairment, the emergence of secondary neoplasms and gonadal dysfunction. BIBLIOGRAPHIC REFERENCES: Aur, R.J.A.: Relapses in children with ALL off treatment. Letter to the Editor. NEJM 292;431, 1975. Simone, J.V., Aur, R.J.A., Hustu, H.O., Verzosa, M. and Pinkel, D.: Combined modality therapy of acute lymphocytic leukemia. Cancer 35:25-35, 1975.